In recent years, in photolithography, along with high integration and high functionality of integrated circuits, microsizing of integrated circuit has been progressing. Accordingly, an exposure device is required to form an image of a circuit pattern on a wafer with a high resolution with a long focal depth, and blue shift of the exposure light source is in progress. The exposure light source has been advanced from the conventional g-line (wavelength: 436 nm), i-line (wavelength: 365 nm) or KrF excimer laser (wavelength: 248 nm), and now an ArF excimer laser (wavelength: 193 nm) is being used. Further, in order to be prepared for an integrated circuit for the next generation where the line width of a circuit pattern will be less than 100 nm, an immersion technique for an exposure system for ArF excimer laser, or a technique for employing a F2 laser (wavelength: 157 nm) as the exposure light source, is being developed. But, it is considered that even these techniques can not cover beyond a generation of a line width of 45 nm.
Under these circumstances, a lithographic technique employing a light having a wavelength of 13.5 nm as a representative example among EUV light (extreme ultraviolet light) as the exposure light source, has attracted attention, as it may be applied to multiple generations of the printing of feature widths of 45 nm and smaller. The image-forming principle of the EUV lithography (hereinafter referred to as “EUVL”) is the same as the conventional photolithography to such an extent that a mask pattern is transferred by means of an optical projection system. However, in the energy region of EUV light, there is no material to let the light pass therethrough. Accordingly, a transmissive optical system can not be used, and all optical systems will be required to be a reflective optical system.
The optical material for the exposure device to be used for EUVL is basically constituted by (1) a substrate, (2) a reflective multilayer film coated on the substrate and (3) an absorber layer formed on the reflective multilayer film. For the multilayer film, it is studied to coat layers of Mo/Si alternately. For the absorber layer, it is studied to use Ta or Cr as the layer-forming material. With regard to the substrate, a material having a low coefficient of thermal expansion is required so that expansion of substrate caused by heat generated by absorption of light will cause no strain even under irradiation with EUV light. In addition, a substrate to be used for EUVL is required to have strict flatness and few defects as compared with a silica glass used for conventional photolithography.
In conventional photolithographic technique employing a transmissive optical system, a silica glass known to have a low coefficient of thermal expansion is used as a substrate. However, the coefficient of thermal expansion (CTE) of the silica glass is high for the optical material for an exposure device to be used for EUVL, and the strain is not negligible under irradiation with EUV light.
On the other hand, a silica glass containing TiO2 (hereinafter referred to as TiO2—SiO2 glass in this specification) is known to be an ultra low thermal expansion material having a coefficient of thermal expansion lower than silica glass. In addition, the coefficient of thermal expansion can be controlled by the TiO2 content in the glass. Therefore, with such TiO2—SiO2 glass, it is possible to obtain a zero expansion glass having a coefficient of thermal expansion being close to zero. Accordingly, TiO2—SiO2 glass is a candidate for an optical material for an exposure device to be used for EUVL.
JP-A-2005-22954 discloses a process of forming a TiO2—SiO2 porous glass body, converting it into a glass body and then obtaining a mask substrate.